It's just that with Spring, I can go to an extremely well-written user manual, or to StackOverflow and get my questions answered. With the Python/Flask codebase, I had to splunk my way through all the layers of random libraries the original developers slapped together, in an attempt to reproduce something resembling out-of-the-box Spring.
I suspect that those original developers had fun making all of those custom choices back in the beginning. I don't know for sure though, since of course they've all left the company since then. The company chose to migrate because it wasn't maintainable once that tribal knowledge left.
I'm sure there are Flask style frameworks in Java land too. And there was a port of Spring for Python a while back too :)
Had to stand-up a server in Java recently.
At some point I realized I was doing more programming via XML files and Spring decorators than via actual Java code. Mostly because Java itself isn't a great abstraction for a CRUD server.
I haven't touched XML for nearly a decade as a full-time Java developer.
BUT I have to say, every project using gradle as alternative so far has receded into bizarre ad-hoc deployment scripting. Maybe that's just because gradle can do stuff that was hidden away in jenkins build files, but still ...
Predictably, the team therefore used both.
I usually enjoy Spark[1] for bootstrapping a simple REST-like interface. There are other options, but in general, you don't really need glue-languages at all if you stray away from old-fashioned EE-style frameworks.